Entropic crystal-crystal transitions of Brownian squares.

When a monolayer of hard microscale square platelets, produced lithographically, is osmotically concentrated in a flat plane to raise the particle area fraction Φ(A), an order-order transition occurs between a hexagonal rotator crystal and a rhombic crystal. Strikingly, phases having fourfold symmetry are not observed at any Φ(A). The rhombic lattice angle α increases continuously with Φ(A), as the system maximizes its total rotational and translational entropy. A cage model, based on packing rotationally swept squares, or "squaroids," reasonably predicts the measured α(Φ(A)), indicating that rotational entropy and the square particle shape combine to produce the rhombic unit cell.